Ultra-high pressure lamp innovations have produced one of the brightest lighting technologies known. Such bulbs are frequently used in projectors and other applications. A quartz bulb typically includes a spherical middle portion from which opposed first and second cylindrical portions extend in a co-axial manner. The spherical middle portion defines an interior chamber which contains mercury and/or halogen vapor or gas. Two electrodes within the chamber are typically made of tungsten or other high-melting point metal. First and second conductors extend from the electrodes through the opposed first and second cylindrical portions. In a typical application, the conductors are made of molybdenum, which is a conducing material that will bond to the quartz used to make the bulb. Such bonding is necessary to prevent leakage of the mercury and/or halogen vapor, particularly at high operating temperatures and pressures.
A failure mode for such bulbs involves degradation of the electrodes due to high start-up voltages. Accordingly, structures have been developed to reduce the start-up voltage required. In one design that reduces the required start-up voltage, one of the cylindrical portions may define a small bubble—within the quartz rod making up the cylindrical portion—which may be filled with mercury and/or halogen vapor. A coil surrounding the quartz rod and bubble is located on one side of the spherical middle portion, while the center of the reflector is located on the other side of the middle portion. The coil is typically held to ground or negative voltage while high voltage is applied to the electrodes. The coil acts in a manner similar to one plate of a capacitor, and tends to assist in the stimulation of the vapor in the bubble, thereby causing UV light to pass into the chamber. The UV light tends to reduce the start-up voltage required to create plasma from the mercury vapor. Upon creation of the plasma, the start-up voltage is stepped down to an operating voltage, and the pressure within the chamber defined within the spherical portion increases to approximately 200 atmospheres.
FIG. 9, Prior Art, shows a cross-sectional view of an exemplary Prior Art lamp 900. A high-pressure bulb 902 is centrally located within a reflector 904. The bulb 902 includes a rearward cylindrical extension 906, which is attached to the reflector 904, typically by adhesive. A forward cylindrical extension 908 is wrapped with a coil 912, which aids in starting the bulb 902. A fireball portion 910 of the bulb 902 is located at approximately a focal point of the reflector 904. A wire 916 provides power to one of the electrodes within the bulb 902. Unfortunately, the coil 912 is sometimes damaged due to its exposed position on the cylindrical portion 908 of the bulb 902. An additional problem is seen in that the coil 912 and wire 914 leading to the coil 912 tend to block light emitted from the spherical portion 910 of the bulb 902. Accordingly, improved ultra high pressure lamps are needed.